For
the Cadillac 4.1L engine there are only two cylinder heads, one that
has a somewhat oval combustion chamber (see illustrations) that is
commonly referred to as the “non-swirl” and produces a published 8.5:1
compression. The second head has a somewhat heart shaped combustion
chamber and is referred to as the “swirl” head and produces 9.0:1
published compression.

When using the non-swirl head in a swirl
application it renders poor drivability issues and if the swirl is
where a non-swirl should be you have detonation issues, neither of
which is acceptable. There are two particular casting numbers that seem
to put the hurt on many. The combustion chamber is always the positive
identifier.

Casting number 1628558 is one of the cylinder heads
that is the “non-swirl” chamber and 1628556 is one of the casting
numbers used for the “swirl” head. Anyone who has spent any time
looking at GM cast iron heads and blocks knows that sixes and eights
have been confused since the ’50s. The numbers are identical other than
that last digit and the digits are either 6 or 8. Other than the
combustion chamber the heads are identical. As you can see casting
proliferation issues began way back in the ’80s and care in
identification is mandatory – not only then but even more so now. The
chart (below) will assist in doing that.